1. Field of Art
This invention relates to laser gyros, and more particularly to improvements in the optical readout thereof.
2. Description of the Prior Art
The differential laser gyro, as described in U.S. Pat. No. 3,862,803, to Yntema et al, includes four modes of traveling waves, two modes being of one polarization and the other two modes being of an orthogonal polarization with respect thereto. The modes relating to each polarization represent an independent laser gyro including both clockwise and anticlockwise circulating waves. The clockwise and anticlockwise waves of one polarization are heterodyned to provide a first gyro output, and the clockwise and anticlockwise waves of the second polarization are heterodyned to provide a second gyro output. The difference between the two gyro outputs provides a differential system output, with twice the normal sensitivity and the cancellation of substantial portions of all biases and errors. As is described in the aforementioned Yntema et al patent, the four modes are caused to operate at four different frequencies by means of a quartz crystal which has an anisotropic frequency response to waves of differing polarization (such as right circularly polarized vs. left circularly polarized), which separates the two distinct gyros (one in each polarization) diversely in frequency from each other, as well as Faraday rotator (or cell) which has an anisotropic frequency response to circularly polarized waves traveling in opposite directions, causing the clockwise and anticlockwise circulating waves in each gyro to be separated by a bias frequency (as is common among nondifferential laser gyros, as well).
Since the Faraday cell requires circular polarization (in contrast with linear polarization) for its anisotropic frequency-splitting effect, it is common to operate laser gyros in circular polarization modes, rather than in plane polarization modes. Due to the inherent requirements for each mode of the circulating waves to have a continuous phase characteristic, there must be an integral number of wavelengths about the closed-loop optical path. Use of an optically active element (e.g., the quartz crystal) will force the modes to be in circular polarization rather than plane polarization because of that fact. Therefore, unless quarter wave plates are used within the laser cavity itself, the only naturally-sustainable modes in such a cavity with an optically active element are the circular polarized modes. It is possible to provide quarter wave plates on either end of the Faraday cell so that circular polarization exists within the Faraday cell with linear polarization in the remainder of the laser gyro. However, due to various perturbating effects on the modes within differential laser gyros, such as the lack of orthogonality of waves if the quarter wave plates are not perfectly aligned, it is preferable to operate laser gyros in the circular polarized modes.
In order to separate one gyro from the other, all four beams or modes (including left circular polarized anticlockwise, right circular polarized anticlockwise, left circular polarized clockwise and right circular polarized clockwise) are coupled out, and the right circularly polarized and left circularly polarized beams are separated from each other to provide distinct gyro outputs by means of polarization filtering. However, as is known, the only practical polarization filter is operative with respect to linear polarization, and not with respect to circular polarization. Therefore, it is common for differential laser gyros to employ quarter wave plates to convert the circularly polarized modes to linear polarized modes prior to separation by polarization filtering. One phenomenon of circularly polarized light is that it changes its orthogonal sense (from left circularly polarized to right circularly polarized, and vice versa) upon each reflection from a mirror. Since provision of output coupling inherently requires the use of mirrors, the polarization reversal must be accommodated so long as circularly polarized light is being coupled toward the output components. Therefore, as illustrated in U.S. Pat. No. 3,892,486 to Ferrar, it is common practice to convert the circularly polarized light to linearly polarized light before encountering the unequal member of reflections of a mode-combining beam splitter, after only an equal number of reflections have been made in the output-coupling beams, so that each independent gyro of a common polarization, including both a clockwise and an anticlockwise circulating wave, can be separated from the other gyro by means of linear polarization filters.
Some representations of laser gyros depict coupling of a portion of each of the four modes from the main laser path to readout components directly through the corner mirrors of the laser gyro itself; in some laser gyro configurations, the readout is achieved by allowing a small percentage to leak through the mirrors; that is, rather than having the mirrors be totally reflective, they are partially transmissive. This operates suitably when the polarization of the modes in the laser gyro are not significant, as in the case of two mode (nondifferential) laser gyros. However, the leakage through a mirror is usually unequal for the horizontal and vertical components of polarization, so any light transmitted through a mirror is necessarily somewhat elliptically polarized, reducing the detectible difference between right circular polarized and left circular polarized waves. Therefore, actual practice in modern differential laser gyros avoids any coupling through the main gyro mirrors since effective gyro operation requires that the mirrors be highly reflective and disposed to enhance gyro operation, rather than to serve the needs of output-coupling. Therefore, modern differential laser gyros employ reflective output coupling, typically deriving the output from slightly tilted surfaces of a component which exists in the gyro path. In such a case, the reflection of the clockwise waves is achieved from a different surface than the reflection of the counterclockwise waves; or if from the same surface, in opposite directions. And beam directors (such as mirrors, or prisms having reflective surfaces) are utilized to direct the beams into colinearity for combining so that the clockwise and counterclockwise waves of each of the two separate gyros can be brought together for detection. An example is illustrated in the aforementioned Ferrar patent.
With readout optics in which the anticlockwise waves of both gyros pass through one quarter wave plate and the clockwise waves of both gyros pass through the other quarter wave plate, perfect alignment of the quarter wave plates is required in order to provide complete orthogonality as between the two separate individual gyros so that complete separation may be made by polarization filtering. Thus, if one of the quarter wave plates is not perfectly orthogonally aligned, some of the left circularly polarized component may leak through, and some of the right circularly polarized component may be blocked (or vice versa), causing each gyro to itself be slightly diminished, and to have, in its output signal, some of the output of the other gyro. This is referred to hereinafter as "cross-talk." Additionally, due to practical limitations on real world laser components (such as the imperfection of mirrors), the circular polarizations are not perfect, but are, rather, elliptical. Adjustment of the quarter wave plates for maximum conversion of the major axis of one of the elliptical polarizations inherently will not suit the major axis of the other elliptical polarization. This there is an inherent cross-talk created when the quarter wave plates are utilized at a point in the readout optics which include just the anticlockwise waves or just the clockwise waves, respectively. The phenomenon is not limited to readout optics of the type schematically illustrated in the Ferrar et al patent, but may also exist in more modern, practical configurations such as the figure eight block configuration disclosed in U.S. Pat. No. 4,000,947 to Grant. Therein, although not described in detail, the quarter wave plates would inherently have to be inserted respectively in each of the clockwise and anticlockwise output beams, between the prisms and the beam splitter. Thus the same problems described hereinbefore with respect to the Ferrar patent will exist in the Grant patent.
A second consideration, illustrated by the readout optics disclosed in the Ferrar and Grant patents, is that the polarization filters are typically aligned mutually orthogonally for the respective separate gyros. This prevents utilization of a single optical output component package for both gyros due to their mutually orthogonal polarizations.